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16 SOCIETY OF VERTEBRATE PALEONTOLOGY. MEMOIR 3 the maxillary nerve (or neurovascular bundle, since vessels accompany the nerve) provides a sensitive guide to the maximal dorsal extent of the muscle. In crocodilians, the dorsal pterygoideus fills the caudal part of the antorbital cavity, and the foramen is dorsally situated. In birds, the dorsal pterygoideus is restricted to the palatine bone at the caudoventral corner of the antorbital cavity, and the maxillary nerve traverses a foramen or gap between maxilla and palatine. Despite the difference in size and apparent position of the muscle, the position of the maxillary neurovascular foramen faithfully indicates the muscle's general location. The Hypothesis Given these correspondences between the components of the extant phylogenetic bracket, we may hypothesize that the common ancestor of Archosauria had a large M. pterygoideus, pars dorsalis that had the following characteristics: (1) it originated from the palatine and pterygoid and probably excavated a fossa on the palatine, and (2) it was situated ventral to the maxillary neurovascular foramen andlor grooves, indicating that the muscle was restricted to the caudoventral portion of the antorbital cavity. This hypothesis is tested by searching for the osteological correlates in the other, extinct descendants of the common ancestor. Testing the Hypothesis There is abundant evidence in most major clades of fossil archosaurs for the presence and general position of a dorsal pterygoideus muscle. As will become apparent, most fossil archosaurs resemble extant birds more than crocodilians. As a result, crocodylomorphs deserve special attention. Crocodylomorpha-In many fossil crocodylomorphs, the prefrontal bone has a transversely broad flange that projects far ventrally into the postnasal fenestra (Fig. 1) diverting any musculature ventrally and creating a cavity rostral to it within the nasoantorbital cavity. This prefrontal flange is found in Dibothrosuchus elaphros (IVPP V 7907; Wu and Chatterjee, 1993), Sphenosuchus acutus (Walker, 1990), Protosuchus richardsoni (UCMP 130860; Clark, 1986), all thalattosuchians examined for this study, and Theriosuchus pusillus (BMNH 48330), among others, and is probably primiti,ve for Crocodylomorpha if not a more inclusive group (it is somewhat developed in the stagonolepidids Desmatosuchus haplocerus [TTUP 90231 and Stagonolepis robertsoni [Walker, 19611, but apparently not in Postosuchus kirkpatricki [TTUP 9000, 90021). In many of these (e.g., Protosuchus richardsoni, Pelagosaurus typus [BMNH 325991, Metriorhynchus superciliosus [BMNH R39001, Theriosuchus pusillus), the prefrontal flange is a delicate, thin plate that appears too fragile to serve as area for adductor muscle attachment; rather, it probably supported the nasal capsule. Thus, the muscle was probably displaced ventrally relative to extant crocodilians. The suborbital fenestra also provides some measure of the rostral extent of the muscle. In basal crocodylomorphs (sphenosuchians and protosuchians), the suborbital fenestra, at most, barely reaches into the antorbital cavity. In Sphenosuchus acutus, there is a distinct muscular fossa on the dorsal surface of the palatine, just rostral to the suborbital fenestra (Walker, 1990: fig. 3b); the area rostral to this crest is probably associated with the nasal cavity in some way. In most mesoeucrocodilians, however, the suborbital fenestra-and presumably the dorsal pterygoideus-is canied farther into the antorbital cavity. In some thalattosuchians (e.g., Metriorhynchus superciliosus, BMNH R2048), there is a tapering fossa or groove on the dorsal surface of the palatine rostral to the suborbital fenestra, whereas in others (e.g., Pelagosaurus typus) there is little direct evidence of the muscle. With regard to the maxillary neurovasculature, Walker (1990) described and figured for Sphenosuchus acutus a large medial foramen and groove in the body of the maxilla just internal to the antorbital fenestra and dorsal to the palatine, suggesting that, as in birds, the muscle was restricted to the caudoventral aspect of the antorbital cavity. Similarly, in the protosuchian Shantungosuchus hangjinensis (Wu, Brinkman, and Lii, 1994) and the thalattosuchians Pelagosaurus typus (BMNH 32599, 32607) and Metriorhynchus superciliosus (BMNH R3900), the maxillary neurovascular foramina enter the medial surface of the bone ventral to the antorbital fenestra. In Sebecus icaeorhinus (AMNH 3160), the neurovascular foramina also enter the maxilla internally just dorsal to the teeth, suggesting limited dorsal extent of the dorsal pterygoideus, which is in agreement with Colbert's (1946a) reconstruction of that muscle. Thus, basally in crocodylomorphs, there is good evidence that the dorsal pterygoideus did not extend beyond the caudoventral portion of the antorbital cavity. Although this study has not sought to determine at which level in Crocodylomorpha the modern condition appeared, it may coincide with loss or transformation of the descending transverse flange of the prefrontal. Alternatively, it may coincide with the appearance of the morphogenetic rotation of the nasal cavity (Witmer, 1995b) that, in extant crocodilians, brings the neurovasculature to a dorsal position in the snout relative to its primitive, more ventral position. Other Crurotarsi-There appear to be two types of putative muscular fossae on the dorsal surfaces of the palatine bones in other crurotarsan archosaurs. In the first type (found also in the crocodylomorph Sphenosuchus acutus [Walker, 1990]), there is a well defined fossa just rostral to and clearly associated with the suborbital fenestra. Stagonolepis robertsoni has this type of fossa, clearly possessing (BMNH R8582) a dorsal excavation of the palatine rostral to the suborbital fenestra that strongly undercuts the rostral portion of the palatine and curves onto the pterygoid and maxilla (Fig. 9C). The rostral portion of the palatine also has a fossa that must be associated with the nasal cavity in some way. It floors the antorbital cavity caudal to the choana in S. robertsoni, but in Desmatosuchus haplocerus (TTUP 9023) this rostral portion of the palatine appears to extend rostrally lateral to the choana to attach to the ascending rarnus of the maxilla, forming a partial rostromedial wall to the antorbital cavity. Furthermore, in Longosuchus meadei, processes from the lacrimal and prefrontal essentially close the postnasal fenestra, preventing any substantial muscular incursion into the antorbital cavity (Parrish, 1994). Thus, stagonolepidids would seem to have had a caudoventrally restricted dorsal pterygoideus. In the second type of muscular fossa on the palatine, the fossa appears to extend rostrally all the way up to the choana, resembling the situation in birds. This condition characterizes many groups of archosaurs and may well be the primitive condition although the situation in many nonarchosaurian archosauriforms is unknown and is equivocal for many archosaurs. Ornithosuchus longidens (BMNH R3143; see also Walker, 1964) has this type of fossa, displaying a large excavation on the dorsal surfaces af the palatine and pterygoid bones extending rostrally up to and undercutting a strong, elevated ridge or strut bordering the choana (Fig. 10C). This postchoanal strut itself is grooved, presumably for attachment of'the cartilaginous nasal ca~sule. Parasuchians also show this condition. usuallv with the maxilla also contributing to the floor of the antorbital cavity. In most parasuchians (e.g., Pseudopalatus pristinus, AMNH 7222; well figured by Case [1929: fig. 161 and Camp [1930: fig. 33]), the palatine and vomer are drawn up dorsally behind the choana into a spout-like structure directed toward the naris. The conjoined transverse postchoanal crests of the vomer and palatine often trend caudolaterally, delimiting a fos-
WITMER-ANTORBITAL CAVITY OF ARCHOSA URS 17 fos antorb lac fos musc max rial fos FIGURE 9. Stagonolepis robertsoni, facial skeleton. A, left lateral view. B, ventral view. C, dorsal view of the palate. (Modified from Walker [I9611 and specimens). sa, here interpreted as a muscular fossa. A final non-dinosaurian example is the poorly known "Pallisteria angustimentum" (BMNH unnumbered; still a nomen nudum, regarded as "Thecodontia incertae sedis" by Carroll [1988]). "P. angustimentum" has a large dorsal fossa on the palatine and pterygoid that extends up to the choana under almost the entire antorbital cavity, suggesting an extensive (or at least long) muscle. Thus, in this latter group, it might seem that because an extensive dorsal pterygoideus was present in much of the antorbital cavity, perhaps it may have even extended beyond the floor and onto the adjacent fossa surrounding the antorbital fenestra. However, the morphological details in these taxa make this unlikely. In Ornithosuchus longidens, for example, the strong postchoanal strut on the palatine extends caudolaterally along its contact with the maxilla, overhanging and providing a rostrolateral border to the muscular fossa (Fig. 10C). Sirnilarly, in "Pallisteria angustimentum," the maxilla projects dorsomedially along its palatine contact, overhanging the muscular fossa. Thus, in these taxa, the muscular fossa was in a topological domain within the antorbital cavity that was separate from the fenestral region. The course of the maxillary neurovasculature is apparent in many of the above forms. In Stagonolepis robertsoni (BMNH R4787), there is a broad groove on the dorsal surface of the max FIGURE 10. Ornithosuchus longidens, facial skeleton. A, left lateral view. B, ventral view. C, dorsolateral view of right side of palate (BMNH R3143). (Modified from Walker [I9641 and specimens.) body of the maxilla medial to the lacrimal and jugal articulations that narrows rostrally as it leads into a foramen located at the mid-length of the internal antorbital fenestra. Farther rostrally, there is another, larger foramen within a fossa in the base of the caudal surface of the ascending process (also visible in BMNH R8582). The poposaurid (or rauisuchid; see Galton, 1985a; Benton, 1986; Parrish, 1993) Teratosaurus suevicus (BMNH 38646) exhibits a large medial foramen in the maxilla within a depression below the antorbital fenestra and dorsal to the palatine contact. Sill (1974) reported and figured for the prestosuchid Saurosuchus galilei a similar medial foramen in the body of the maxilla, as did Dutuit (1979) for an unnamed Moroccan form. For Prestosuchus chiniquensis, Azevedo (1995) did not discuss the specific skeletal features of interest here but reconstructed the dorsal pterygoideus as being restricted to the caudoventral portion of the antorbital cavity, well away from the internal antorbital fenestra. Among parasuchians, specimens referred to Phytosaurus cylindricodon (BMNH 38039, 38040) and Rutiodon carolinensis (AMNH 4) exhibit medial foramina leading into large canals running within the maxillae just ventral to the antorbital fenestra and probably ex-
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